Thanks for your adding to my confusion
Actually, quite reassuring though.
As far as I can see no formal BEA membership in the Drift Group. According to the acknowledgements, BEA paid for some of the work of the group's members and there was a BEA administrator involved in keeping the Drift Group on "a focussed track" (p 144), which could be perceived as very funny were it not that it concerns such a serious occasion.
Dutch